The present invention generally relates to a method for correcting the pointing of a satellite antenna. More particularly, the present invention relates to a method for correcting the pointing of a satellite multiple aperture, multiple beam antenna using an earth-based beacon received by a satellite-based tracking feed for each antenna aperture.
Satellite-based communications systems are highly reliant upon accurate antenna orientation. Due to the considerable distance between an orbiting satellite and the earth, a flawed orientation of only a few degrees (or in some cases much less than one degree) may have a great impact on overall system performance. Imperfections in orientation or alignment may arise from a number of factors including subtle imperfections in manufacture, mechanical stresses placed on the system components while in orbit or during entry into orbit, and the stresses inherent in the orbital environment, such as solar-induced heat fluctuations, for example.
In cellular satellite-based communications systems, antenna pointing is particularly important. The satellite antenna must be accurately pointed because of the narrow beamwidth used to service each communication cell. Typically, in modern systems, each communications cell is serviced by an antenna beam with a 0.7 degree half-power beamwidth. Thus, in systems of this type, a pointing error of only a fraction of a degree may have a considerable impact on system performance and consequently system revenue. Additionally, cellular satellite-based communications systems are typically configured so that a single antenna structure may service several cells within the area of coverage. The specific cells serviced by the antenna are sometimes referred to as a service pattern, a frequency reuse pattern, a cellular coverage pattern, or simply an antenna pattern. Thus, a pointing error in a single antenna may adversely impact many cells within the communications area.
FIG. 1 illustrates a typical cellular communications system 100 in which a pointing error has occurred. The cellular communications system 100 includes a satellite 110 carrying an antenna 120 which generates numerous beams 130. Each beam is directed towards a specific cell in an earth-based cellular coverage pattern 150, shown as solid circles. The beams 130 generated by the antenna 120 are designed to provide communications with several communication cells (160–166). However, due to the pointing error of the antenna 120, the actual area of coverage for the beams is shown by the off-pointed spot beams (170–176) with coverage areas indicated by dashed circles.
Because the spot beams are not correctly aligned with the cellular coverage pattern 150, several problems arise. For example, if the antenna is an uplink antenna, the user terminals may need to provide additional power to communicate successfully with the satellite 110. Additional gain may increase costs, power consumption, and the system's overall interference level, thus decreasing performance.
Alternately, if the antenna is a downlink antenna, additional transmit power may be necessary at the satellite. However, this additional power may interfere with communications in other cells, quickly drain the limited power reserves onboard the satellite, and increase the cost, weight, and complexity of the satellite.
In a typical cellular communications system, the uplink communications beams and the downlink communications beams are generated at separate antennas. Different antennas are used because the uplink and downlink take place in different frequency ranges and the antenna size is adapted to the frequency of communication. For the reasons explained above, both uplink and downlink antennas must be accurately pointed.
An economic and operative premium is placed on the ability of the satellite to accurately point antennas, particularly antennas such as Multiple Beam Antenna (MBA) apertures that provide service for many cells simultaneously. Consequently, satellite spacecraft and antenna designers in the past were forced to develop and implement extremely accurate attitude control subsystems and very accurately aligned and thermally stable structures to maintain acceptable antenna pointing. The result was significant additional cost and complexity in the satellite design, construction, and operation.
Typically, when a satellite is installed in orbit, the pointing of the satellite is carefully adjusted. However, faults in installation or the stress of operation may induce a pointing error. While the satellite may be equipped with an attitude control system allowing the satellite to adjust its position relative to the earth (and thus the pointing of the antennae) adjustments in attitude may provide too coarse an adjustment. Additionally, adjusting the pointing of a single antenna through attitude control may induce a pointing error in other antennas mounted on the satellite.
Satellite antennas are often implemented as a reflector such as a Casegrain reflectors system or an offset parabolic reflector, for example. Thus, often in the implementation of a satellite communication system, the pointing of the reflectors is adjusted during installation only and the system is made to perform regardless of pointing error and the corresponding communications degradation. This is the “fix and forget” method of installation.
However, more advanced satellites have been constructed with antenna gimbals which allow the pointing of the antenna to be adjusted independent of the attitude of the satellite. The gimbal is mounted on the satellite and provides mechanical rotation for the antenna around the mounting point of the gimbal. Typically, in these cases, the antenna is a dish reflector with a center mounted gimbal.
Although gimbaling of a reflector allows the reflector to be independently adjusted and re-pointed, the correct pointing of the reflector must still be determined. One method of pointing the reflector is to measure the uplink power from the received users in the communication cell. If the uplink power of each user is measured, then the reflector can be positioned to maximize the average power of the users. While this method may be somewhat effective if the users are evenly spread throughout the cell, this method does not yield an accurate pointing of the reflector if the users are located near the cell edge. Additionally, interference and environmental and atmospheric effects may impact the receive signal power and cause further pointing error. Furthermore, measuring the received power from users and determining the maximum power positioning of the satellite adds additional complexity, cost, and weight to the satellite.
Thus, a need has long existed for increased accuracy in the pointing of satellite antenna, particularly in a cellular communications satellite system.